The proposed research investigates the process by which young immigrants in the US learn English as their second language (L2) and the factors that facilitate or impede this process. The study is a continuation of the on-going research project that is currently supported by a SCORE grant (#413531117) awarded to the PI for the period of 2001-2003. The major goals are to provide a more comprehensive description of the L2 acquisition process in the immersion setting, to identify predictive variables of L2 proficiency, and to develop a descriptive and an explanatory account of the L2 acquisition process. Specifically, the study will include one group of native Spanish speakers (n=160) and one group of native Mandarin Chinese speakers (n = 160) of various ages (7-18 years) who are at different stages of their English acquisition (having been residing in the US from 1 to 5 years). Their proficiency in English phonology, lexicon, and morphosyntax, as well as first language (L1) proficiency in lexicon and morphosyntax will be assessed. A set of predictive variables of L2 proficiency will also be examined, such as age of L2 immersion, length of US residence, and various aspects of language environment. The study will also include 96 English monolingual controls divided into six chronological age groups (7-8, 9-10, 11-12, 13-14, 15-16, 16-18 years). The proposed research is unique in its coverage of an extended initial period of L2 immersion, its simultaneous assessment of proficiency in both L1 and L2 and in multiple linguistic components, and its investigation of a comprehensive set of possible contributors to L2 acquisition. The work has important potential applications for L2 acquisition and diagnosis of speech/language problems among children with limited English proficiency. The work will also use L2 acquisition as a vehicle to illuminate related issues central to the sciences of human development and cognition, such as the maturation of the human language learning mechanism).